1. Field of the Invention
The present invention relates generally to a wheel alignment control system for vehicles, which controls a wheel alignment factor of the vehicle. More particularly, the invention relates to a wheel alignment control system for a vehicle having at least one pair of road wheels.
2. Description of Relevant Art
In general, in a vehicle having at least one pair of left and right road wheels, the wheels have a factor or factors of wheel alignment thereof preset to provide the vehicle with favorable travelling characteristics.
The term "wheel alignment" generally refers to the alignment of wheels and is usually used, in the art of vehicles of the type which have at least one pair of road wheels, as a term generically naming those angular relations which the wheels have relative to a body of such vehicle or the ground. Practically, the wheel alignment is represented by a number of well-known geometrical factors such as kingpin offset, kingpin inclination, trail, caster, camber, and toe-in.
Such factors will be briefly described below, as well as mutual relations thereamong and influences thereof on travel-related characteristics as of a vehicle in the art in concern, whereas for easy understanding such vehicle is assumed throughout the description to be standing still on a flat ground surface, with a steering wheel put in a neutral position thereof.
For each road wheel with a kingpin, the factor "kingpin offset" is defined as a distance on the ground surface in contact with a tire of the wheel, from the center of a grounded portion of the tire to the axial centerline of the kingpin, as it is viewed from ahead of a body of the vehicle; "kingpin inclination", an angle that, when viewed from ahead of the vehicle body, the axial centerline of the kingpin forms with the vertical; and "trail", a distance in the longitudinal direction of the vehicle body, between the center of the wheel and a point at which the axial centerline of the kingpin intersects with the ground surface. Each of these three factors depends on the geometrical arrangement of the kingpin and has an effect on the steering characteristic as well as on the force necessary for a driver to turn the steering wheel.
In this respect, the kingpin has a moment produced thereabout by longitudinal components of external forces acting on the grounded portion of the tire. Such moment becomes large, as the kingpin offset has a larger value, with the possibility of exerting a substantial influence on the steering characteristic. For such reason, there has been recent tendency to set the kingpin offset as small as possible.
The "caster" factor of the wheel alignment is defined, for each road wheel with a kingpin, as an inclination angle of the axial centerline of the kingpin as viewed from either side of the vehicle body. The caster is provided to set the aforesaid trail.
The "camber" factor of the wheel alignment is defined, for each road wheel, as a tilt angle that the center plane of the wheel as viewed from ahead of the vehicle body has with respect to a vertical plane extending in the longitudinal direction of the vehicle body, the vertical plane passing through the center of a grounded portion of a tire of the wheel, and assigned to have a positive value when, as viewed from the driver's position, the wheel is tilted outward from the vertical plane, and a negative value when the wheel is tilted inward therefrom.
Usually, the camber is set positive to minimize the kingpin offset. However, when set positive, the camber of any road wheel produces a force which tends to cause the wheel to roll out of the advance direction thereof while the vehicle is travelling, that is, what is called camber thrust.
To compensate for the camber thrust, each pair of such wheels as opposed to each other in the transverse direction of a vehicle are generally provided with the "toe-in" factor of the wheel alignment. For such paired wheels, the toe-in is defined as the difference of a distance b between the respective rear ends of the wheels as viewed from thereabove minus a distance a between the front ends thereof. In this respect, the wheels are said to be forced to toe in when the rear end distance b is larger than the front end distance a, and toe out when the distance b is smaller than the distance a.
In case, for a pair of road wheels, the camber is set negative, a camber thrust is exerted on each wheel in a direction opposite to that in the case of a positive camber angle, thus biasing the wheel inside of the advance direction thereof. In such case, the wheels should be forced to toe out, to thereby compensate for the camber thrust.
Moreover, for a pair of wheels forced to toe in or out, when representing the quantity of toe-in or toe-out by a toeing angle of the wheels to be positive or negative, respectively, it is known that the toeing angle varies in accordance with the compliance, that is, an elastic deformation of an associated suspension system, which variation accounts for a vehicle characteristic called compliance steer.
With respect to the foregoing description, in conventional vehicles of the type in question, the wheel alignment as well as the compliance characteristic of a suspension is preset to be substantially fixed, in spite of having essential effects on the steering characteristic.
Under certain conditions, however, for a vehicle of the type in question, it is desirable to set a pair of road wheels thereof for a negative camber angle, or in other words, to make each thereof tilted inward from the vertical plane, at vehicle speeds exceeding a predetermined value. For example, when the vehicle as thus set is turned around a corner, a limitation to the travelling characteristic thereof may be relaxed all the more for the camber angle to be then varied close to a zero degree.
To the contrary, as will be understood from the foregoing description, in case the camber angle is left set negative while the vehicle is travelling at extremely low speeds, the paired wheels may exert an interrepulsive thrust on each other to the extent that can result in an increase in the abrasion of tires as well as an increase of fuel cost.
Likewise, in a vehicle of the type in question, for stabilization of the travelling characteristic thereof at relatively high vehicle speeds it is desirable to set a pair of road wheels thereof for a relatively large toe-in quantity, which imparts to each wheel a tendency roll inside of the advance direction thereof, thus permitting the wheel to be aligned all the better in the advance direction at such vehicle speeds where the wheel itself is caused to slip out on a road surface to some extent. However, in case the toe-in quantity is left unchanged from such setting, an increased tire abrasion as well as an increase in fuel cost may result while the vehicle is travelling at extremely low speeds where the wheels seldom tend to slip out.
Incidentally, for a pair of road wheels of a vehicle in the art in question, it is also known that the toeing angle as well as a toeing direction thereof has a substantial effect on the steering characteristic of the vehicle. More particularly, it usually so follows to some extent that the vehicle tends to understeer when the wheels are forced to toe in, and to oversteer when they are forced to toe out.
In conventional vehicles in the art, the wheel alignment is likely to be preset so as to make the steering characteristic understeer to some extent, to thereby stabilize the travelling characteristic at relatively high vehicle speeds. In this respect, in a high-speed region, the travelling charactertistic becomes more stable, as the degree of understeer becomes stronger, whereas in a low-speed region, as this degree becomes weaker, the more the vehicle turnability will be facilitated. The understeering characteristic of the vehicle thus has a contradictory phase between high-speed and low-speed regions. For such reason, in the conventional vehicles, the degree of understeer is generally set to be moderate, that is, at a compromise point, without excessively inclining to the stronger side and without being biased to the weaker side. As a result, for such a vehicle having the degree of understeer thereof preset at such a compromise point, it is difficult to provide an optimum characteristic with respect to both the travelling stability in a high-speed region and the vehicle turnability in a low-speed region.
The present invention has been achieved to effectively overcome such shortcomings of a conventional vehicle.